Resolve printing uint64 types

Using %llu or %lu will break one of the build targets. The "right way"
to print a uint64_t is via the PRIU64 macro defined in C99.

This wasn't defined for the NuttX compiler so it was added to px4_defines.h
for NuttX.

Signed-off-by: Mark Charlebois <charlebm@gmail.com>

src/platforms/px4_defines.h

@@ -105,6 +105,10 @@ typedef param_t px4_param_t;
 
 #define PX4_ISFINITE(x) isfinite(x)
 
+#ifndef PRIu64
+#define PRIu64 "llu"
+#endif
+
 /* 
  * POSIX Specific defines
  */